RFID tags are rapidly becoming quite important for tracking and identifying goods as well as for identifying customer accounts. Small tags having a transponder chip and antenna offer many advantages over simple bar codes, including unique serialization, non-contact reading through an outer packaging material, and on-chip storage of information for some transponder chip versions. RFID tags have proven themselves to be quite useful in a wide variety of applications, including those such as bin identification, pallet identification, product serialization, access card identification, and account identification.
Just as RFID tag application breath is wide, so also is the environment in which the tags are read. Thus the kind of transceiver antenna that is appropriate for reading tags on a pallet of goods passing through a doorway is different from the kind of transceiver that may be appropriate for reading a patron's account information at a vending machine. The antenna for reading tags on a pallet of goods may be a pair of wire loops two feet wide by four feet tall, one on each side of the pallet when it is in position to be read. Conversely, the antenna for reading a patron's key-fob RFID tag may be single sided, just a few square inches in size at most, and have a correspondingly shorter reading range.
Generally, RFID readers are fairly large and separate from any associated display of the information transmitted or received. Placing display circuitry in close proximity to an RFID transceiver antenna could adversely interact with the antenna by reducing the Q (quality factor) of its resonance through coupling the transmitted energy into the display circuitry resulting in energy loss from the tuned antenna circuit. The Q of an antenna is roughly proportional to both the radiated signal strength and receiver sensitivity, both of which are important for increasing the reading range to an RFID tag. Additionally, a high Q antenna implicitly means that it is narrow band and susceptible to the possibility that metal in the local vicinity may change the tuning of the central resonant frequency of the antenna away from the operating frequency of the RFID system thus degrading the reading range to an RFID tag. The operating frequency of a tuned antenna is inversely proportional to the square root of the antenna's inductance and thus is directly affected by metal objects within the radiation pattern of the antenna. Eddy currents may flow in the metal object as a result of a mutual inductance coupling term between the antenna and the metal object, thus altering the net inductance of the antenna and correspondingly altering the center frequency of the tuned antenna. In order to mount a small RFID reader antenna with an integrated visual display through a metal panel while maintaining its Q and center frequency requires a design that considers and avoids the aforementioned problems.
Mounting an industrial inductive proximity sensor through a metal panel has analogous problems to that of the RFID reader and similarly requires the need for immunity of the sensor to surrounding metal. An inductive proximity sensor having a shielded pot core configuration sensing surface and an indicator LED at the opposite end of its tubular enclosure is disclosed in U.S. Pat. No. 6,229,420 granted May 8, 2001 to Bauml, et al.
A fueling transaction system using RFID tags for customer account identification at the pump is disclosed in U.S. Pat. No. 6,116,505 granted Sep. 12, 2000 to Withrow wherein it is described how communications between the transceiver antenna and transponder tag require the absence of metal objects coming between them and thus when antennas are mounted within the fueling dispenser, glass or plastic dispenser walls are preferable.
An RFID reader having a cylindrical housing with a coil wound ferrite rod core that includes a light emitting diode indicator and a piezo buzzer on the reader's front face is disclosed in U.S. Pat. No. 5,378,880 granted Jan. 3, 1995 to Eberhardt. The disclosure is devoid of any discussion of the effects that the light emitting diode indicator, piezo buzzer, or a metal panel mounting location may have on the Q or center frequency of the antenna.
A multi-directional RFID read/write antenna unit in an industrial proximity sensor housing having a plurality of coils adapted to transmit multi-directional RF signals to an RFID tag and receive RF responses therefrom is disclosed in U.S. Pat. No. 6,069,564 granted May 30, 2000 to Hatano, et al. wherein each of the coils is ferrite shielded from the others and has no means for visual indication integrated with any of the sensing surfaces.
A Metal compensated RFID reader housed so that the influence of metallic objects in its physical surroundings on system performance is minimized by using a pre-compensation metal plate to stabilize the self-resonant frequency of the reader is disclosed in U.S. Pat. No. 6,377,176 granted Apr. 23, 2002 to Lee. There is no means for visual indication integrated with the sensing surface.
A bridge circuit utilizing a pair of back-to-back pot core sensors operating at 10 KHz to provide positive identification of a metal body is disclosed in U.S. Pat. No. 4,847,552 granted Jul. 11, 1989 to Howard. There is no means for visual indication integrated with the sensing surface.
Despite the considerable effort that has been applied heretofore in the design of RFID transceivers none have produced a compact RFID reader that can be mounted through a metal panel and integrate status indication into the sensor face without having the antenna be adversely affected by the presence of the status indicator within the transmitted field or adversely affected by the proximity of the metal in a panel when being mounted therethrough. Many applications for RFID validation are considerably space limited. Manufacturers of equipment that use RFID validation would prefer no restrictions on the materials they use to produce their products just because they wish to install an RFID reader. Finally, many applications for RFID validation do not have other suitable displays available to indicate the status of the sensor or of the information transacted and must rely on a status indicator integrated into the reader.
As can readily be appreciated, there remains a need for further improvement in the features and operation of RFID readers, and in particular RFID readers offering a small footprint that can be mounted through a metal panel and provide status indication integrated with the sensing surface.